Manufacturing polysilicon through chemical vapor deposition of silicon in a fluidized bed reactor (FBR) has advantages over the traditional Siemens process, which decomposes silane or chlorosilane onto silicon “slim” rods in a bell-jar reactor. With reference to FIG. 1A, which shows a specific design of one type of FBR reactor, at least one divider 102 divides a reactor chamber 100 into a pre-reaction heating zone 104 and a reaction zone 106. In the specific example of FIG. 1A, the divider is a cylindrical, vertical divider 102 so that the zones 104, 106 are concentric. Small “seed” particles of polysilicon are introduced through a particle inlet port 110 into the pre-reaction heating zone 104, where they are heated by a heater 108 to a temperature above the silicon deposition temperature of a reaction gas comprising at least one silicon-containing reagent. A pre-reaction fluidizing gas that does not contain silicon, typically hydrogen gas, is introduced through a pre-reaction gas port 112 into the pre-reaction heating zone 104, where it fluidizes the silicon particles.
The flow rate of the pre-reaction fluidizing gas is adjusted to a level that allows the silicon particles to slowly fall out of the pre-reaction heating zone and into the bottom of the reaction zone 106. There, they are fluidized by the reaction gas, which is introduced into the reaction zone 106 through a reaction gas port 114. Due to the high temperature of the silicon particles, the reaction gas is decomposed upon contact with the particles, and silicon is deposited onto the particles, causing them to grow. Eventually, the particles become too heavy for the reaction gas to lift, and they fall out of the reactor through an exit port 118. In some cases, such as the example given in FIG. 1A, the reaction gas is able to lift at least some of the silicon particles above the top of the separator 102, so that they fall back into the heating zone 104. This causes particles that cool before they are heavy enough to fall out of the reaction zone 106 to be re-circulated through the heating zone 104.
Silicon FBR technology has been industrialized for years. The advantages of FBR reactors, as compared to Siemens reactors, for silicon production include low energy consumption and continuous operation. In addition, the silicon product is in granular form, which can be handled readily in downstream processes for the making of silicon ingots and single crystals.
However, several problems still exist which affect the process and product quality. One of these problems is dust formation. As the reaction gas is heated by the seed particles, the temperature of at least some reaction gas molecules may rise above their decomposition temperature, even when they are not in direct contact with a silicon particle. As a result, molecules in the reaction gas can spontaneously decompose, forming very fine particles of silicon “dust.” Dust formation in an FBR reactor is not only a waste of silicon, but is also hard to handle, due to its low density. When attached to the surfaces of the silicon product particles, dust also degrades the quality of the silicon product.
Another problem in conventional FBR reactors is deposition of the reaction gas on the vertical separator 102 and/or the walls of the FBR chamber 100. One way to heat up the particles in an FBR is to heat the walls of the pre-reaction heating zone 104 with external heaters, as shown in FIG. 1A, so that the walls transfer heat by conduction to the particles within the pre-reaction heating zone 104. In these configurations, the walls of the pre-reaction heating zone are hotter than the particles, and if any reaction gas reaches them, the result will be deposition of silicon on the walls. When the silicon layer on the walls gets too thick, the FBR operation must be stopped to allow removal of the silicon from the walls.
A third problem with conventional FBR reactors is the high concentration of hydrogen typically found in the granular polysilicon product. Hydrogen gas is often used as the pre-reaction fluidizing gas that is introduced through gas port 112, and hydrogen gas is also frequently a component of the reaction gas. Hydrogen trapped within and/or adsorbed onto the surface of product silicon particles can result in bubbles in the melting process downstream, which can be detrimental to the quality of ingots or crystals grown from the melt. Therefore, the hydrogen must first be removed from the granular silicon particles produced by an FBR reactor, before the silicon can be used. A typical approach is to dehydrogenate the silicon particles by transferring them to a separate dehydrogenation zone that is distinct from the FBR, where the particles are fluidized by hydrogen gas, or by some other dehydrogenation gas, and heated to a high temperature that drives off the hydrogen. However, this approach requires a separate process and separate equipment, which makes the process more complex, and increases both the capital and operating costs.
It is important to note the direction in which the silicon particles circulate in the FBR of FIG. 1A. This circulation direction is illustrated more clearly in FIG. 1B. The silicon particles move downward as they are heated in the pre-reaction heating zone 104 until they enter the bottom of the reaction zone 106, where they come into contact with the reaction gas. As a result, the silicon particles come into initial contact with the reaction gas in a region where the silicon concentration is highest, and the particles are at their maximum temperature. This accelerates the initial rate of silicon deposition, and reduces the amount of time that the particles must remain in the reactor, on average, before being removed as silicon product. The particles are then carried upward by the reaction gas, and those particles that do not receive sufficient silicon to fall out of the reactor through the exit port 118 are carried over the vertical separator 102 to once again fall downward and be heated in the pre-reaction heating zone 104.
Unfortunately, due to the higher flow rate of the reaction gas, which carries the particles upward, as compared to the pre-reaction fluidizing gas, which allows the particles to drift downward, it is inevitable that some of the reaction gas will also flow over or under the vertical separator 102 and thereby enter into the pre-reaction heating zone, where it will readily form silicon dust and deposit silicon on the heated walls.
What is needed, therefore, is a polysilicon FBR reactor that produces less silicon dust, minimizes deposition of silicon on the walls, heaters, and zone dividers, and produces silicon product with reduced hydrogen content, so that a separate dehydrogenation step is not needed.